Thermodynamic scales for sulfur atom transfer and oxo-for-sulfido exchange reactions

Abstract

This review is intended as a resource for persons interested in the synthesis of new, interesting molecular sulfides, both organic and inorganic. The approach to this goal has been to organize sulfur atom donor/acceptor reagents onto a reactivity scale that identifies, at least in thermodynamic terins, the relative strengths of potential donors and acceptors. This review has also attempted to call attention to sulfur atom donor agents that are potentially very useful but somewhat outside the usual repertoire of reagents that are employed. Chief among these are hexasulfur (S6), dialkyl dithiiranes, sulfur diimides, bis(tri-n-butylstannyl)sulfide, and 5-aryloxy1,2,3,4-thiatriazoles. Similarly, oxo-for-sulfido exchange reagents such as SiS2, Ph3SiSH, or Lawesson's reagent (for inorganic synthesis) have been little used but may be cleaner, milder, more selective, or more convenient alternatives in chalcogenide exchange reactions than B2S3 or Me3SiSSiMe3. The absence of many of these potentially useful reagents from the thermodynamic scales (Tables 1-6) identifies areas where experimental thermochemistry can contribute useful new information. The determination of standard molar enthalpies of formation for dialkyl dithiiranes (and also for thiones and thioaldehydes), for sulfur diimides, and for 5-aryloxy-1,2,3,4-thiatriazoles would be most helpful in quantifying their sulfur atom donor activity. Other areas of sulfide chemistry also represent opportunites for research. For instance, terminal sulfido complexes of ruthenium and osmium do not appear to be known, probably in part because no determined effort has been directed toward synthesizing such compounds. Terminal sulfido complexes of manganese and chromium are also unknown but pose greater synthetic challenges owing to the lesser stability of these first-row metals in the MIV oxidation state that would be minimally required to support a terminal sulfido ligand. Organic sulfides such as C60S and acenaphthalene sulfide have not yet been prepared and isolated but perhaps could be with the right choice of sulfur atom donor and reaction conditions. Catalytic sulfur atom transfer systems that extend and improve upon the pioneering work of Adam, for instance, by activating sluggish substrates such as cyclohexene or by performing enantioselective sulfur atom transfer to prochiral molecules, are also wide open areas for new chemical research.